Half-bridge circuits employing normally on switches and methods of preventing unintended current flow therein

ABSTRACT

A method for rendering a half-bridge circuit containing normally on switches such as junction field effect transistors (JFETS) inherently safe from uncontrolled current flow is described. The switches can be made from silicon carbide or from silicon. The methods described herein allow for the use of better performing normally on switches in place of normally off switches in integrated power modules thereby improving the efficiency, size, weight, and cost of the integrated power modules. As described herein, a power supply can be added to the gate driver circuitry. The power supply can be self starting and self oscillating while being capable of deriving all of its source energy from the terminals supplying electrical potential to the normally on switch through the gate driver. The terminal characteristics of the normally on switch can then be coordinated to the input-to-output characteristics of the power supply.

BACKGROUND

1. Technical Field

This application relates generally to half-bridge circuits employingnormally on switches and to electrical devices which include one or moresuch circuits.

2. Background of the Technology

Silicon Carbide (SiC), a wide band-gap semiconductor material, is veryattractive for use in high-power, high-temperature, and/or radiationresistant electronics. SiC power switches are logical candidates forthese applications due to their excellent material physical propertiessuch as wide energy band-gap, high breakdown field strength, highsaturated electron drift velocity and high thermal conductivity comparedto the conventional silicon counter part. In addition to the aboveadvantages, SiC power devices can operate with much lower specificon-resistance than conventional silicon power devices [1]. Because ofthese properties, SiC unipolar devices are expected to displace siliconbipolar switches (such as the insulated gate bipolar transistor or IGBT)and rectifiers in the 600-3000 V range.

A necessary component to the value proposition that the market demandsto adopt the new SiC switch technology is found in the lower specific onresistance of SiC switches. A factor of ten or more reduction isinherent to SiC devices, but only in the form most likely to producethese specific on-resistance reductions. The SiC junction field effecttransistor (JFET) has demonstrated the lowest specific on-resistance ofany SiC switch. In particular, the normally on version of this devicewill minimize the specific on-resistance at any voltage rating. Themarket place, however, tends to prefer normally off devices because theyare perceived as being inherently safe when imbedded in an application.

An independent trend in the market place is to package powerful switchesinto modules that typically consist of two, four, or six switchesarranged in parallel half-bridge configurations. These modules areparticularly useful for the control of rotating machinery in devicesknown as motor drives, the operating principals of which are well known[2, 3]. FIG. 1 illustrates a single half-bridge module containing twosilicon IGBT switches. Typical ratings for the switches in the modulesare 600-V or 1200-V blocking potential and hundreds of amperes ofconduction (e.g., 300-A, 400-A, or 600-A) but the voltages and currentscan be higher or lower. The half-bridge interconnection and theconstituent IGBT both have weaknesses. The former requires the addedcomplication of an isolated gate driver for the high-side switch. Thelatter has a gate terminal that can be easily damaged by incorrect orexcessively stressful control potentials. The latter also has many otherfailure modes which designers must avoid when designing applications,principally by making design tradeoffs in the circuitry known as thegate drive. The market has responded by including imbedded gate drivercircuitry into the switch modules so that the application external tothe module does not actually apply potentials to the gate terminals ofthe IGBTs. This additional level of safety has dramatically increasedthe reliability of IGBTs used in the field.

There still exists a need, however, for improved circuits and deviceswhich modulate the flow of electrical current from one or moreelectrical sources to one or more electrical loads.

SUMMARY

According to a first embodiment, a circuit is provided which comprises:

-   -   a first normally-on switch having a source, a drain and a gate,        the source of the first normally-on switch electrically        connected to a load and the drain of the first normally-on        switch electrically connected to a supply voltage;    -   a second normally-on switch having a source, a drain, and a        gate, the source of the second normally-on switch electrically        connected to a common voltage and the drain of the second        normally-on switch electrically connected to the load;    -   a first gate driver electrically connected between the gate and        the source of the first normally-on switch;    -   a second gate driver electrically connected between the gate and        the source of the second normally-on switch; and    -   a first oscillating power supply electrically connected in        parallel across the drain and the source of the first switch,        the first power supply configured to power the first gate driver        such that, for a given voltage applied to the input to the first        power supply, the first power supply generates an output voltage        which can be supplied to the gate of the first switch by the        first gate driver;    -   a second oscillating power supply electrically connected in        parallel across the drain and the source of the second switch,        the second power supply configured to power the second gate        driver such that, for a given voltage applied to the input to        the second power supply, the second power supply generates an        output voltage which can be supplied to the gate of the second        switch by the second gate driver.

According to a second embodiment, a circuit is provided which comprises:

-   -   a first normally-on switch having a source, a drain and a gate,        the source of the first normally-on switch electrically        connected to a load and the drain of the first normally-on        switch electrically connected to a supply voltage;    -   a second normally-on switch having a source, a drain, and a        gate, the source of the second normally-on switch electrically        connected to a common voltage and the drain of the second        normally-on switch electrically connected to the load;    -   a first gate driver electrically connected between the gate and        the source of the first normally-on switch;    -   a second gate driver electrically connected between the gate and        the source of the second normally-on switch; and    -   an oscillating power supply electrically connected across the        voltage source and the common voltage, wherein the power supply        is configured to power the first and second gate drivers such        that, for a given voltage applied to the input of the power        supply, the power supply generates a first output voltage which        can be supplied to the gate of the first switch by the first        gate driver and a second output voltage which can be supplied to        the gate of the second switch by the second gate driver.

According to a third embodiment, an electrical device comprising atleast one circuit as set forth above is also provided.

According to a fourth embodiment, an electrical device comprising threecircuits as set forth above arranged in parallel wherein the device is athree-phase motor drive is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a half-bridge switch module consisting of asilicon IGBT (normally off high-side switch) in series with a secondIGBT (normally off low-side switch). The module shown in FIG. 1 can alsoinclude an anti-parallel diode rectifier (not shown). Isolated powersupplies can be used to derive negative and positive potentials to drivethe insulated gate of the switch.

FIG. 2 is an example of a half-bridge switch module comprising anormally on switch such as a junction field effect transistor (JFET) inseries with a second normally on switch. The module shown in FIG. 2 canalso include an anti-parallel diode rectifier (not shown). Isolatedpower supplies can be used to derive negative and positive potentials todrive the insulated gate of the switch.

FIG. 3 is a schematic diagram of a power half-bridge circuit comprisinga normally on junction field effect transistor (JFET) made from siliconcarbide in the high and low side switch positions. As shown in FIG. 3, apower supply is connected across the input to the drain and sourceterminals of each JFET. The input-to-output characteristics of the powersupply can be coordinated with the blocking characteristics of the JFET.

FIG. 4 is a schematic diagram of a power half-bridge circuit wherein theinput to a single power supply is connected from the upper bus to thelower bus of the half-bridge module. This power supply derives twoisolated outputs, one for the high-side JFET and one for the low-sideJFET. The input-to-output characteristics of the power supply can becoordinated with the blocking characteristics of the JFET.

FIG. 5 is a graph showing coordination between the input-outputcharacteristic of the power supply and the blocking characteristic ofthe corresponding normally on switch (e.g., JFET) protected by the powersupply.

FIG. 6 is a further illustration of the coordination between theinput-to-output characteristic of the power supply and the family ofcurves of the normally on JFET while the JFET is in the passive currentlimiting mode.

FIG. 7 is a schematic diagram of one physical implementation of thepower supply based on a non-isolated Cuk converter that is connectedacross the drain-to-source terminals of the protected JFET.

FIG. 8 is a plot of the experimental data measured using the physicalimplementation shown in FIG. 7. In the figure the data marked “required”refers to the experimental blocking characteristic curve of the SiC JFETbeing controlled by the gate drive. The data marked “derivedexperimentally” is the input-to-output characteristic of the powersupply shown in FIG. 7.

DETAILED DESCRIPTION

A method for rendering the use of normally on switches inherently safewhen packaged in a module consisting of one or more switches that are inturn arranged and controlled so as to modulate the flow of electricalcurrent from one or more electrical sources to one or more electricalloads is described. The switches can be made from a semiconductormaterial such as silicon or silicon carbide. The normally on switch canbe a junction field effect transistor (JFET), but the methods are notlimited to such switches. Any normally on switch that exhibits aproportional relationship between the potential applied across thecontrol terminals of the device and the potential that the device canblock across the controlled terminals of the device without the flow ofexcessive leakage current can be rendered safe by the methods describedherein.

A described herein, a circuit can be added to the gate drive circuitryof each switch in the module such that a normally on switch can have thesame inherent safety, from the point of view of the external terminalsof the module, that is usually available only from a normally offswitch. In this way, better performing normally on switches can be usedin the module without compromising the safety, or the perception ofsafety, in the eyes of the application designer. Furthermore, thecircuitry can be coordinated with the physical properties of the switchto ensure a safe (i.e., non-damaging) response to any potential appliedto the switch. An exemplary, non-limiting example of the normally onswitch is a silicon carbide power JFET.

As set forth above, a method for rendering a half-bridge circuitcontaining normally on switches, such as junction field effecttransistors (JFETs) made, for example, from silicon carbide or fromsilicon, inherently safe from uncontrolled current flow (i.e., “shootthrough”) is provided. The methods described herein allow for the use ofbetter performing normally on switches in place of normally off switchesin integrated power modules thereby improving the efficiency, size,weight, and cost of such integrated power modules.

As set forth herein, a power supply may be added to the gate drivercircuitry. The power supply may be self starting and self oscillatingwhile being capable of deriving all of its source energy from theterminals supplying electrical potential to the normally on switch. Sucha configuration is common to a wide class of devices known as integratedpower modules. The methods described herein, however, are not limited tointegrated power modules. Rather, these methods are applicable to anysystem consisting of one or more half-bridge circuits. A common exampleis the three-phase electric motor drive, which uses three half-bridgecircuits in parallel.

As also described herein, the terminal characteristics of the normallyon switch can be coordinated to the input-to-output characteristics ofthe power supply. The details of the coordination and the results from aworking model are described below.

An object of the present invention is to illustrate a means forrendering normally on power semiconductor switches with the sameinherent safety against potentially damaging unintended flow ofelectrical current that is provided by the normally off switches.

A further object of the invention is to describe the interconnection ofadditional circuitry in the gate driver portion of the typical powerswitch module that provides one aspect of the means for renderingnormally on switches inherently safe.

A further object of the invention is to describe the coordinationbetween the properties of the additional circuitry and the properties ofthe normally on power switch that results in the inherently safe design.

A further object of the invention is to describe the constraints thatdefine the “crossover” between active current limiting, which isprovided by the additional circuitry mentioned above, and passivecurrent limiting, which is provided by the normally on power switchitself.

As described herein, the result of these various objects is a means ofrendering the normally on power switch inherently safe that is robust,technically feasible, and consistent with current and future designpractice in power modules.

The benefits of the current invention are considerable. For example,better performing normally on power switches can be used in powermodules, especially when the switches are made from silicon carbide,which will shrink the size and weight of these modules and make themmore efficient. In addition, it is likely that more applications forpower modules will become possible because of the faster switchingperformance typical of unipolar devices over bipolar devices.

The present invention will be described in greater detail hereafter withreference to the accompanying drawings, in which a preferred embodimentof the invention is described with silicon carbide (SiC) normally onJunction Field Effect Transistors (JFETs).

By “normally on” it is meant a transistor switch that will conductsignificant current when no voltage or current bias is applied to thegating terminal of the transistor.

Silicon carbide crystallizes in more than 200 different poly-types. Themost important are: 3C—SiC (cubic unit cell, zincblende); 2H—SiC;4H—SiC; 6H—SiC (hexagonal unit cell, wurtzile); and 15R—SiC(rhombohedral unit cell). However, the 4H— polytype is more attractivefor power devices thanks to its larger bandgap and higher electronmobility. Although 4H—SiC is preferred, it is to be understood that thepresent invention is applicable to devices made of other poly-types ofsilicon carbide or made from elemental silicon, by way of example.

Referring to FIG. 1, an example schematic of a two-switch half bridgecommonly used by itself as a power processor in numerous powerelectronic applications (e.g., dc-dc converter [1] and dc-ac inverters[2]) or as a building block for three-phase inverters which typicallyrequires three half-bridge circuits in parallel [3]. The switchesillustrated in FIG. 1 are normally off insulated gate bipolartransistors (IGBT). In modules spanning the range of voltage ratingsfrom 600 V to thousands of volts and spanning the range of currentratings from 50 A to thousands of amperes, the silicon IGBT is mostcommon. While a negative gate potential is not generally required tocause the IGBT switch to be in the non-conducting state, FIG. 1 includesthe possibility of a negative potential (−V_(SS) in FIG. 1) because thisis known to improve the efficiency of switching certain types of IGBTsin applications [4].

New switch technology in the form of normally on SiC JFETs haveexhibited the lowest specific on-resistance and the lowest gate chargefigures of merit among power semiconductor devices with voltage ratingssubstantially above 300 V, and thus have the potential to displacesilicon IGBTs in many of their current and future applications [5]. Theadvantages of using a unipolar device, like the SiC JFET, over a bipolardevice, like the Si IGBT, can accrue benefits such as better switchingresponse, lower forward conduction loss, and relaxed thermal management.In contrast, traditional silicon unipolar devices such as the MOSFETsimply cannot deliver low enough forward conduction loss to compete withthe silicon IGBT, especially at voltages above 600-V. Non-traditionalnormally on silicon and silicon carbide unipolar devices arecompetitive, the latter extremely so.

As illustrated in FIG. 2, a normally on silicon JFET (also known as thestatic induction transistor or SIT) has been used to make powerinverters [6]. However, for these devices the negative potentialrequired to pinch-off a normally on switch and thus render it in thenon-conducting state is derived from the power supply powering the gatedriver in a manner similar to that commonly provided for normally offswitches. The disadvantage of this approach is that if the source ofisolated power is removed (e.g., by accident or by failure) then themeans for supplying the necessary negative potential is also removed.This can leave the half bridge made from normally on switches in anuncontrolled conducting state which can severely damage thesemiconductor switches and/or the external load or source by permittinguncontrolled and excessive electrical current flow (sometimes referredto as “shoot through”). This concern, above all, has favored the siliconIGBT over the silicon JFET in virtually all practical applications.

Referring now to FIG. 3, a schematic block diagram is shown for a powerhalf-bridge circuit comprising a normally on junction field effecttransistor (JFET) made from silicon carbide in the high and low sideswitch positions. The provision of negative gate potential is no longerdependent on an isolated power source, although one can still beprovided as a means of redundancy. Instead, or in addition, a powersupply is connected across the drain and the source terminals of thenormally on switch (e.g., a JFET as depicted in FIG. 3). This powersupply can have input-to-output characteristics which are coordinatedwith the electrical characteristics of the JFET in a manner describedbelow. This coordination can be used to ensure that, for any combinationof electrical potentials applied across the drain and source terminalsof the JFET, an uncontrolled electrical current cannot flow through theJFET. In FIG. 3, two power supplies are shown, one for each switch ofthe half bridge. This arrangement provides maximum protection to eachJFET.

FIG. 4 illustrates an alternative embodiment in which a single powersupply with appropriately coordinated input-to-output characteristics isused to derive the negative potential for both JFETs in the half bridge.The power supply in the device shown in FIG. 4 is isolated. The devicedepicted in FIG. 4 can be used in applications where the source ofpotential that can be applied to the JFETs in the half bridge is limitedto the upper and the lower bus connections spanned by the half bridge.The configuration shown in FIG. 3 can be used in all power moduleapplications. The device shown in FIG. 4 is suitable for many commonapplications and has the advantage of requiring only one power supplywhich may reduce cost and part count.

The power supply illustrated in FIG. 3 and FIG. 4 derives power tooperate from the source of potential applied to either the drain-sourceterminals (FIG. 3) or the upper and lower buses (FIG. 4). In addition,the power supply depicted in FIGS. 3 and 4 will start oscillatingautomatically upon application of a voltage to its input.

In addition, the input-to-output characteristics of the power supply canbe coordinated with the semiconductor device or devices being protected.For example, the voltage that a vertical power JFET will block can berepresented on a graph as illustrated in FIG. 5. On the horizontal axisor abscissa is plotted the potential applied across the gate to thesource of the JFET. On the vertical axis or ordinate is plotted thepotential applied across the drain to the source of the JFET when thespecified leakage current flows into the drain. The solid curve on thegraph traces the blocking characteristics of the JFET. The gate-sourcevoltage can be positive or negative, but V_(DS)@I_(D)=I_(leakage)increases monotonically as the gate-source voltage decreases. IfV_(GS)>V_(threshold) and I_(D) is limited to ≦I_(leakage), then noappreciable V_(DS) can be blocked. The maximum voltage that the JFET canblock is given by BV_(DSS) which occurs when V_(GS)=V_(GS(off)). A“normally on device” is generally one in which V_(threshold)<0. Eventhough it is a normally on device, it may not reach its full ability toconduct (i.e., exhibiting the minimum resistance to electrical current)until V_(GS)>0.

Superimposed on the graph in FIG. 5 is a dashed line which representsthe input-to-output characteristics of the self-oscillating powersupply. When a certain V_(DS) is applied to the input of the powersupply, it will generate an output that is applied to V_(GS) by the gatedriver circuit. The locus of ordered pairs of V_(GS) and V_(DS) isrepresented by the dashed line. According to some embodiments,coordination can be used to ensure that the dashed line falls below thesolid line for the majority of applied V_(DS). For the region where thisis true, labeled “active current limiting” on the graph, it is certainthat the applied V_(DS) will produce a leakage current through the JFETthat is less than I_(leakage). In other words, over the range of V_(GS)corresponding to active current limiting the JFET is safely in itsnon-conducting state, which means the current flowing through the JFETwill be very small and inconsequential. The cross over voltage isdefined as the boundary value of V_(GS) separating the active currentlimiting mode from the passive current limiting mode.

When V_(GS) is greater than the cross over voltage, the JFET is at leastpartly passively limiting the current. If the gate driver is notactively commanded by an external controller to bias the JFET to minimumon-resistance (i.e., turning the switch “ON”), then the gate driverdesign will inherently cause V_(GS)=−V_(SS)≦0. The design of gate drivecircuits satisfying this requirement is simple and reliable, and knownto those skilled in the art. When the JFET is passively limiting thecurrent, it is dissipating more energy than when it is actively limitingthe current. The locus of V_(GS) vs. V_(DS) produced by the power supplycan be such that, when V_(GS) is greater than the cross over voltage,the JFET will not overheat. To those skilled in the art, this means thatthe JFET remains within its safe operating area (SOA) at all times.

FIG. 6 illustrates the general condition under which safe passivecurrent limiting will be achieved. The graph plots the JFET's draincurrent (I_(D)) on the ordinate and the drain-source voltage (V_(DS)) onthe abscissa. A family of curves is drawn on the graph by graphing theordered pairs of I_(D) vs. V_(DS) for a constant V_(GS), and thenincrementing V_(GS) and redrawing the I_(D) vs. V_(DS) curve. In FIG. 6it is assumed that V_(GS) ranges from positive values to negative valuesand includes V_(GS)=0 V. The last curve plotted is that forV_(GS)=V_(threshold). V_(GS)=0 V is another curve indicated on thegraph. An example of a family of curves for a SiC JFET might cover −4V≦V_(GS)≦+3 V. The more negative that V_(GS) becomes, the more resistivethe JFET becomes, which reduces I_(D) at any given V_(DS). As long asthe gate driver is not commanded to switch the JFET ON, it willquiescently clamp V_(GS) to −V_(SS) (i.e., V_(GS)=−V_(SS)). Since themaximum value of −V_(SS)=0, then it is not possible for I_(D) to fall ona line above the V_(GS)=0 V curve. In practice, even a modest V_(DS)will cause −V_(SS)<0, thus I_(D) must always fall on a line below thatbounded by the V_(GS)=0 V curve. This limiting load line is plotted onthe graph in FIG. 6 as a dashed line. By definition, the safe operatingarea (SOA) of a useful power switch will be bounded at low V_(DS) by therated maximum allowable current of the switch. This is also true for theJFET. In turn, the maximum allowable current will fall on the minimumresistance curve. In the case of a typical SiC JFET, the minimumresistance curve will occur well above the V_(GS)=0 V curve. Forexample, in the range 0 V<V_(GS)≦+3 V, as is illustrated in FIG. 6. As aresult it is not possible while the gate driver clamps V_(GS)=−V_(SS)for the drain current through the JFET to exceed that permitted by theSOA. In other words, while in the passive current limiting mode thecurrent flowing through the JFET may not be very small, but it will beinconsequential from the standpoint of damaging the JFET provided thatthe application designer follows accepted design principals to ensurethe application operates the JFET within its SOA.

Referring now to FIG. 7, a schematic diagram of an exemplary andnon-limiting embodiment of a self oscillating power supply isillustrated. This circuit is the Cuk converter useful for converting aninput voltage into an output voltage that is smaller in magnitude andopposite in polarity. The power supply shown in FIG. 7 is non-isolated.It is therefore suitable for use in the circuit illustrated in FIG. 3.

As illustrated in FIG. 8, the power supply shown in FIG. 7 accepts apositive input voltage ranging from 0 to 600 V and converts it into anegative output voltage ranging from 0 to −50 V. The cross over voltagefor this power supply occurs at approximately V_(GS)=−5 V which easilysatisfies the safe operating area criteria for a SiC power JFET pairedwith the power supply.

While the foregoing specification teaches the principles of the presentinvention, with examples provided for the purpose of illustration, itwill be appreciated by one skilled in the art from reading thisdisclosure that various changes in form and detail can be made withoutdeparting from the true scope of the invention.

REFERENCES

-   [1] Issa Batarseh, Power Electronic Circuits. Wiley, ISBN    0-471-12662-4, 2004, pp. 224-225.-   [2] Issa Batarseh, Power Electronic Circuits. Wiley, ISBN    0-471-12662-4, 2004, pp. 426-427.-   [3] N. Mohan, T. M. Undeland, and W. P. Robbins, Power Electronics.    Wiley, ISBN 0-471-58408-8, 1995, p. 225.-   [4] S. Abedinpour and K. Shenai, “Insulated Gate Bipolar    Transistor,” Power Electronics Handbook, ed. M. Rashid. Academic    Press, ISBN 0-12-581650-2, 2001, pp. 109-110.-   [5] M. S. Mazzola, L. Cheng, J. Casady, D. Seale, V. Bondarenko, R.    Kelley, and J. Casady, “Scalable SiC Power Switches for Applications    in More Electric Vehicles,” Proc. of 6^(th) Int. All Electric Combat    Vehicle Conf, AECV 2005, Bath, England 13-16 Jun. 2005.-   [6] H. Akagi, T. Sawae, and A. Nabae, “130 kHz 7.5 kW current source    inverters using static induction transistors for induction heating    applications,” IEEE Trans. Power Electronics, vol. 3, no. 3, pp.    303-309, 1988.

1. A circuit comprising: a first normally-on switch having a source, adrain and a gate, the source of the first normally-on switchelectrically connected to a load and the drain of the first normally-onswitch electrically connected to a supply voltage; a second normally-onswitch having a source, a drain, and a gate, the source of the secondnormally-on switch electrically connected to a common voltage and thedrain of the second normally-on switch electrically connected to theload; a first gate driver electrically connected between the gate andthe source of the first normally-on switch; a second gate driverelectrically connected between the gate and the source of the secondnormally-on switch; and a first oscillating power supply electricallyconnected in parallel across the drain and the source of the firstswitch, the first power supply configured to power the first gate driversuch that, for a given voltage applied to the input to the first powersupply, the first power supply generates an output voltage which can besupplied to the gate of the first switch by the first gate driver; asecond oscillating power supply electrically connected in parallelacross the drain and the source of the second switch, the second powersupply configured to power the second gate driver such that, for a givenvoltage applied to the input to the second power supply, the secondpower supply generates an output voltage which can be supplied to thegate of the second switch by the second gate driver.
 2. The circuit ofclaim 1, wherein the input-output characteristics of the first powersupply are coordinated with the characteristics of the first switch suchthat the first switch is in active current limiting mode over at least50 percent of a voltage range of 0 to the breakdown voltage of the firstswitch applied to the first power supply and wherein the input-outputcharacteristics of the second power supply are coordinated with thecharacteristics of the second switch such that the second switch is inactive current limiting mode over at least 50 percent of a voltage rangeof 0 to the breakdown voltage of the second switch applied to the secondpower supply.
 3. The circuit of claim 1, wherein the input-outputcharacteristics of the first power supply are coordinated with thecharacteristics of the first switch such that the first switch is inactive current limiting mode over at least 75 percent of a voltage rangeof 0 to the breakdown voltage of the first switch applied to the firstpower supply and wherein the input-output characteristics of the secondpower supply are coordinated with the characteristics of the secondswitch such that the second switch is in active current limiting modeover at least 75 percent of a voltage range of 0 to the breakdownvoltage of the second switch applied to the second power supply.
 4. Thecircuit of claim 1, wherein the input-output characteristics of thefirst oscillating power supply are coordinated with the characteristicsof the first switch such that the first switch does not overheat when inpassive current limiting mode and wherein the input-outputcharacteristics of the second oscillating power supply are coordinatedwith the characteristics of the second switch such that the secondswitch does not overheat when in passive current limiting mode.
 5. Thecircuit of claim 1, wherein the first gate driver and the second gatedriver each comprise a totem pole driver.
 6. The circuit of claim 1,wherein the first normally-on switch and the second normally-on switchare JFETs.
 7. The circuit of claim 1, wherein the first oscillatingpower supply and the second oscillating power supply are each aself-oscillating power supply.
 8. The circuit of claim 1, wherein thefirst oscillating power supply and the second oscillating power supplyare each non-isolated.
 9. The circuit of claim 6, wherein the firstnormally-on switch and the second normally-on switch are each SiC JFETs.10. The circuit of claim 8, wherein the first oscillating power supplyand the second oscillating power supply are each Cuk converters.
 11. Anelectrical device comprising at least one circuit as set forth inclaim
 1. 12. An electrical device comprising three circuits as set forthin claim 1 arranged in parallel, wherein the device is a three-phasemotor drive.
 13. A circuit comprising: a first normally-on switch havinga source, a drain and a gate, the source of the first normally-on switchelectrically connected to a load and the drain of the first normally-onswitch electrically connected to a supply voltage; a second normally-onswitch having a source, a drain, and a gate, the source of the secondnormally-on switch electrically connected to a common voltage and thedrain of the second normally-on switch electrically connected to theload; a first gate driver electrically connected between the gate andthe source of the first normally-on switch; a second gate driverelectrically connected between the gate and the source of the secondnormally-on switch; and an oscillating power supply electricallyconnected across the voltage source and the common voltage, wherein thepower supply is configured to power the first and second gate driverssuch that, for a given voltage applied to the input of the power supply,the power supply generates a first output voltage which can be suppliedto the gate of the first switch by the first gate driver and a secondoutput voltage which can be supplied to the gate of the second switch bythe second gate driver.
 14. The circuit of claim 13, wherein theinput-output characteristics of the power supply are coordinated withthe characteristics of the first and second switches such that the firstand second switches are in active current limiting mode over at least 50percent of a voltage range of 0 to the breakdown voltage of the firstand second switches applied to the power supply.
 15. The circuit ofclaim 13, wherein the input-output characteristics of the power supplyare coordinated with the characteristics of the first and secondswitches such that the first and second switches are in active currentlimiting mode over at least 75 percent of a voltage range of 0 to thebreakdown voltage of the first and second switches applied to the powersupply.
 16. The circuit of claim 13, wherein the input-outputcharacteristics of the power supply are coordinated with thecharacteristics of the first and second switches such that the first andsecond switches do not overheat when in passive current limiting mode.17. The circuit of claim 13, wherein the first gate driver and thesecond gate driver each comprise a totem pole driver.
 18. The circuit ofclaim 13, wherein the first normally-on switch and the secondnormally-on switch are JFETs.
 19. The circuit of claim 13, wherein theoscillating power supply is a self-oscillating power supply.
 20. Thecircuit of claim 19, wherein the oscillating power supply is isolated.21. The circuit of claim 18, wherein the first normally-on switch andthe second normally-on switch are each SiC JFETs.
 22. An electricaldevice comprising at least one circuit as set forth in claim
 13. 23. Anelectrical device comprising three circuits as set forth in claim 13arranged in parallel, wherein the device is a three-phase motor drive.24. The circuit of claim 1, further comprising an external controllerwhich is adapted to bias the first switch to minimum on-resistance viathe first gate driver.
 25. The circuit of claim 24, wherein the firstgate driver is adapted to apply a bias voltage to the gate of the firstswitch (V_(GS)) which is equal to the output from the first power supply(−V_(SS)) when the external controller is not biasing the first switchto minimum on resistance.